Process and equipment for the conveyance of powdered material

ABSTRACT

The invention concerns a process and a device ( 2 ) for the pneumatic conveyance of powdered material ( 4 ), in which a cylindrical chamber ( 10, 12 ), which can be connected with a reservoir ( 6 ) by a sealable inlet ( 14, 16 ) and with a delivery line ( 28 ) by a sealable outlet ( 18, 20 ), is alternately filled with material from the reservoir ( 6 ) and emptied of this material by applying a negative pressure to the chamber with its outlet ( 18, 20 ) closed and its inlet ( 14, 16 ) open through a bordering wall formed by a gas-permeable filter element ( 50 ) to draw material into the chamber ( 10, 12 ) from the reservoir ( 6 ), and by then admitting a gas under pressure into the chamber ( 10, 12 ) with its inlet ( 14, 16 ) closed and its outlet ( 18, 20 ) open to force the material previously drawn into the chamber ( 10, 12 ) out of the chamber and into the delivery line ( 28 ). To prolong the service life of the filter element ( 50 ) and to avoid contamination of the filter element more easily, it is proposed, in accordance with the invention, that the filter element ( 50 ) be designed as a hollow cylinder and that it surround at least a portion of the chamber ( 10, 12 ).

RELATED APPLICATIONS

The present application is a divisional of pending U.S. Ser. No.11/533,519 filed on Sep. 20, 2006, for PROCESS AND EQUIPMENT FOR THECONVEYANCE OF POWDERED MATERIAL, which is a divisional of U.S. Ser. No.10/501,693 filed on Oct. 20, 2004, for PROCESS AND EQUIPMENT FOR THECONVEYANCE OF POWDERED MATERIAL, now U.S. Pat. No. 7,150,585, which is anational phase entry under 35 U.S.C. §371 and claims priority toInternational Application No. PCT/EP03/10857, with an InternationalFiling Date of Oct. 1, 2003, for PROCESS AND EQUIPMENT FOR THECONVEYANCE OF POWDERED MATERIAL, which claims priority to German patentapplications DE 102 47 829.3, filed on Oct. 14, 2002 and DE 102 61053.3, filed on Dec. 24, 2002, which are all fully incorporated byreference herein.

The invention concerns a process and device for conveying powderedmaterial. The process and device are used especially in powder coatinginstallations to convey powder coating material by compressed air indense phase from a reservoir into a delivery line and through this to aspray gun or other type of spray application device.

Until now, in powder coating installations, the powder coating materialwas usually pneumatically conveyed by the dilute phase method from areservoir to a spray gun through a hose-like delivery line. However,this causes problems, because, first, relatively large amounts ofcompressed air are needed, second, the diameter of the hose-likedelivery line must be relatively large, and third, wear occurs at thebends in the delivery lines. For this reason, over the past severalyears, tests have been conducted at a number of powder coatinginstallations with so-called plug flow conveying or dense phaseconveying, in which the powder coating material is cyclically conveyedeither by gravity or by negative pressure into a chamber and thendischarged from the chamber with compressed air and conveyed in the formof successive “plugs” through the delivery line to the spray applicationdevice.

A process and device of the type cited at the beginning for the plugflow or dense phase conveyance of powdered materials with suction bynegative pressure is already known from DE 196 43 523, DE 196 54 649,and EP 0 937 004 B1. The previously known device has a cylindrical pumpchamber, which is equipped with a discharge opening for the conveyedmaterial at its lower end and a plate-like filter element that isimpenetrable by the conveyed material at its upper end, by which thepump chamber can be alternately connected with a vacuum pump and with asource of compressed gas, to suck the conveyed material from a reservoirthrough a connection that opens into the pump chamber from the side forthe purpose of filling the pump chamber, or to push the conveyedmaterial through the discharge opening into a discharge line for thepurpose of emptying the pump chamber. To allow exact metering of theconveyed material and at the same time a high discharge volume, the pumpchamber should have a filling volume that is as small as possible, andit should be possible to fill it and empty it in an operating cycle thatis as short as possible. To achieve the latter objective, however, thegas must be sucked from the pump chamber and fed into the pump chamberrelatively quickly, which requires a pressure difference that is aslarge as possible between the inside of the pump chamber and the sourceof negative pressure or the source of compressed gas. However, a highpressure difference at the filter element causes larger bending loadsand pressure loads on the filter element and thus a reduction of itsservice life, which is why the filter element must be supported withsupport lattices or the like. However, this in turn results in areduction of its passage cross section, which makes it necessary tochoose between a greater load and thus a shorter service life of thefilter element and a higher gas throughput and thus a shorter operatingcycle. In the case of the pneumatic conveyance of powder coatingmaterial, there is the additional consideration that this material has aparticle size of <80 .μm, and about 10-15% of this material is in theparticle size range of <5 μm. Since this is of the same order ofmagnitude as the pore diameter of the filter materials that are used,small particles can penetrate deeply into the filter element or evenpass through it. Some of the particles that were mentioned first areretained in the filter element during the subsequent admission ofcompressed gas and may not become detached from the filter element againuntil after a prolonged period of time, which can lead to contaminationof the coated surface after a color change. The particles mentioned lastcan cause pump damage, at least when diaphragm pumps are used to producethe negative pressure. However, the use of a smaller pore diameter toavoid these problems would in turn result in a lower gas throughput andthus longer operating cycles. Furthermore, in the previously knowndevice, when the chamber is emptied, residual pigmented powder coatingmaterial can also remain inside the suction intake connection, which canalso result in contamination of the powder coating material during acolor change.

Proceeding on the basis of this prior art, the object of the inventionis to improve a process and device of the type specified at thebeginning in such a way that the service life of the filter element isprolonged and contamination of the filter element can be more easilyavoided.

In accordance with the invention, this object is achieved by designingthe gas-permeable filter element as a hollow cylinder in such a way thatit forms a part of the cylindrical surface of the chamber, unlike thefilter element of the state of the art, in which it is designed as anend wall of the chamber. The invention is based on the idea that thismeasure provides a simple means of increasing the filter surface of thefilter element and thus reducing the pressure difference between theouter and inner surface of the filter element at the same gasthroughput, without increasing the volume of the chamber and thusadversely affecting the metering precision. Furthermore, as a filterelement constructed as a section of the cylindrical wall, it allowsaxial conveyance of the powdered material through the chamber in astraight line, which ensures frictionless conveyance, unlike thesituation with a filter element installed on an end face. Moreover, nomoving parts are required other than at the chamber inlet and outlet.

It is advantageous for the hollow-cylindrical filter element to be madeof a sintered material, preferably a sintered plastic powder, since,when rigid filter materials of this type are used, the use of a supportmaterial becomes unnecessary. In addition, a cylindrical filter elementmade of a rigid filter material has greater stability than a flat filterelement of the same size and therefore can be produced with a smallerwall thickness at equivalent permeability. It is advantageous for thepore diameter of the sintered material to be smaller than the smallestparticle diameter of the powdered material being conveyed, i.e., in thecase of the conveyance of powder coating material, preferably smallerthan 5 μm.

To ensure uniform admission of negative pressure and compressed gas, itis advantageous to surround the filter element with a housing, which isseparated from the filter element by a cylindrical annular space. Theannular space can be alternately connected with a negative pressuresource and a compressed gas source, either by a single connection or,preferably, by two connections, one of which is located in the vicinityof its end face facing the outlet and can be acted upon by negativepressure, while the other is located in the vicinity of its end facefacing the inlet and can be acted upon by compressed gas. The latterarrangement allows a considerable increase in conveyance capacity due tomore rapid and complete filling of the chamber and due to more rapid andcomplete emptying of the chamber.

A further increase in conveyance capacity can be achieved by selectingan optimum ratio between the length and the inside diameter of thehollow-cylindrical filter element, which preferably should be in therange of 10-30.

Tests have shown that relatively short operating cycles of less than 0.5s between two successive intake operations are possible withoutimpairing the service life of the filter element, if the latter extendsover more than one third of the length of the chamber between the inletand the outlet and preferably has a length that is about half the lengthof the chamber.

Another preferred embodiment of the invention provides that the sectionsof the cylindrical wall of the chamber that are adjacent to the ends ofthe filter element are designed to be elastic, and that the inlet andoutlet are located in the vicinity of the elastic cylindrical wallsections and are closed gas-tight by a pneumatically operated pinchvalve.

The inlet and outlet of the chamber are preferably located at theopposite ends of the chamber, so that, when a single chamber is used,which is alternately acted on by compressed gas and negative pressure,the conveyance route can pass through the chamber in a straight line,i.e., without turns or bends. The smallest possible pressure drop alongthe conveyance route is also achieved, if the filter element and theother chamber have an inside diameter that is essentially the same asthe inside diameter of a feed line between the reservoir and the chamberand the inside diameter of the delivery line, so that there are no largechanges in the cross section of the conveyance route.

In accordance with another preferred embodiment of the invention, thecompressed gas is fed into the chamber during the conveyance of thepowdered material through the filter element to remove powdered materialadhering to the inner side of the filter element that faces the chamber.However, due to triboelectric charging of the powdered material duringconveyance, electrostatic adherence of powder particles can also occurelsewhere in the chamber. The surge of compressed gas through the filterelement may not always be sufficient to clean off this powder.Therefore, this cleaning is effectively accomplished by passingcompressed air past the filter element by feeding it into the chamberthrough a cleaning valve.

To prevent powdered material from escaping from the chamber through thiscleaning valve when compressed gas is being admitted into the chamberduring conveyance through the filter element, the cleaning valvepreferably has a diaphragm, which is elastically deformed during thefeeding of compressed gas through the cleaning valve and opens an intakeport for the compressed gas into the chamber, and which, aftercompletion of the feeding of compressed gas through the cleaning valve,is restored to its original shape, in which it tightly seals the intakeport. The diaphragm may consist, for example, of an elastic rubber hosecoupling, which is drawn over the circumferential wall of a pipeconnection, which is closed at its free end and whose circumferentialwall is provided with openings. During the feeding of compressed gasinto the inside of the pipe connection, the elastic rubber hose couplingis lifted from the circumferential surface of the pipe connection by thepressure of the gas, so that the compressed gas can flow between thediaphragm and the circumferential wall into the chamber.

While it would basically be possible to use a diaphragm pump to producethe negative pressure necessary to draw powdered material into thechamber, another preferred embodiment of the invention provides for theuse of a so-called vacuum injector, which is acted upon by compressedair from the compressed air source and produces a negative pressure bythe venturi principle.

To achieve an even faster operating cycle, it is advantageous, as isalready well known, to provide the device with two chambers, one ofwhich is always being filled, while the other is being emptied. In thiscase, the two chambers are preferably aligned parallel to each other andconnected by Y-shaped sections of line with a common feed or conveyanceline, such that each of the angles between the leg and the two arms ofthe Y and each of the angles at the ends of the arms at the transitionto the parallel chambers is preferably less than 30° in order to ensurematerial conveyance that is as unobstructed as possible and undergoeslow pressure losses.

To simplify the design of the device with a double chamber, the inletsand outlets of both chambers are opened and closed with only two closuremechanisms, one of which closes the inlet of the first chamber andsimultaneously opens the inlet of the second chamber, and the otheropens the outlet of the second chamber and simultaneously closes theoutlet of the first chamber. It is advantageous for each of the twoclosure mechanisms to comprise a double-acting pneumatic cylinder withtwo oppositely directed piston rods, whose free ends press against theelastic wall of the adjacent chamber to close its inlet or outlet.

The pneumatic cylinders are preferably controlled by two electromagneticmultiple-way control valves in their compressed air feed lines, so thatit is possible to open the inlet and outlet of a chamber at the sametime for cleaning purposes, for example, to blow out this chambertogether with the feed line and the conveyance line from the reservoir.

Another simplification of the design of the double-chamber device ispossible by using a single four-way or five-way control valve forapplying negative pressure in one chamber and simultaneously admittingcompressed gas into the other. Of its two or three inlets, one isconnected with a compressed air source, and the other or others areconnected with a negative pressure source, which is preferably designedas a vacuum injector, while each of its two outlets is connected withone of the chambers and is alternately connected with the compressed airinlet or one of the negative pressure inlets by switching the valve.

The invention is explained in greater detail below with reference tospecific embodiments shown in the drawings.

FIG. 1 shows a top view of the device of the invention for the pneumaticconveyance of powder coating material by the dense phase process withtwo conveying chambers.

FIG. 2 shows a partially cutaway and somewhat schematic view of thedevice of FIG. 1.

FIG. 3 shows an enlarged longitudinal section of part of one of the twoconveying chambers of the invention.

FIG. 4 shows a simplified pneumatic circuit diagram of part of thedevice.

FIG. 5 shows a sectional view of a preferred negative pressure generatorof the device.

FIG. 6 shows a longitudinal section of an alternative cleaning valve forfeeding compressed air for cleaning into the conveying chambers.

FIG. 7 shows a top view of another device of the invention, whichcorresponds to FIG. 1.

FIG. 8 shows a view of the device in FIG. 7, which corresponds to FIG.2.

FIG. 9 shows an enlarged longitudinal section of part of one of the twoconveying chambers of the device in FIGS. 7 and 8, which corresponds toFIG. 3.

FIG. 10 shows a simplified pneumatic circuit diagram of part of thedevice in FIGS. 7 and 8.

The devices 2 shown in the drawings of FIGS. 1, 2, 7, and 8 are used forthe pneumatic conveyance of powder coating material 4 for theelectrostatic powder coating of objects. The powdered material isconveyed in dense phase from a reservoir 6 to a spray gun 8.

The devices 2 have two parallel cylindrical conveying chambers 10, 12,whose oppositely open ends form an inlet 14, 16 or an outlet 18, 20 forthe powder coating material 4 that is being conveyed and are connectedwith a Y-shaped section 22 of a feed line 24 leading to the reservoir 6and with a Y-shaped section 26 of a flexible delivery line 28 leading tothe spray gun 8.

Each of the two conveying chambers 10, 12 can be closed near its inlet14, 16 and its outlet 18, 20 by a pinch valve 30. The pinch valves 30for the two inlets 14, 16 and the pinch valves 30 for the two outlets18, 20 are operated by two double-acting pneumatic cylinders 32 locatedbetween the two conveying chambers 10, 12. Each of the two pneumaticcylinders 32 has two piston rods 34, which extend from opposite ends.Their spherically rounded free ends 36 at the inlet 14, 16 or the outlet18, 20 engage an elastically deformable, pliable inner wall section 42of the chamber 10, 12 through recesses 38 in a rigid cylindrical outerwall section 40 of the adjacent conveying chamber 10, 12. This pressesthe inner wall section 42 against an opposite part of the chamber walland seals the inlet 14, 16 or the outlet 18, 20 airtight. Except in thearea of the recesses 38, the elastic inner wall sections 42 are firmlyconnected with the rigid sections of the outer wall to prevent them fromcontracting when a negative pressure is applied in the chamber 10, 12.

The two pneumatic cylinders 32 are each connected by an electromagneticmultiple-way control valve 44, 46 (FIGS. 1 and 7) with a compressed airtank 48 (FIGS. 4 and 10). During the conveyance operation, the twocontrol valves 44, 46 are always operated in such a way that thepneumatic cylinders 32 are acted upon crosswise by compressed air, sothat in one chamber 10, the inlet 14 is open and the outlet 18 isclosed, while in the other chamber 12, the inlet 16 is closed and theoutlet 20 is open, or vice versa (as shown in FIGS. 2 and 8).

To allow powder coating material 4 to be drawn from the reservoir 6 intothe chamber 10 or 12 with the open inlet 14 or 16 and to be dischargedfrom the chamber 12 or 10 and into the delivery line 28 with the outlet20 or 18 open, the two chambers 10, 12 are each provided with ahollow-cylindrical filter element 50, which is permeable to air andimpermeable to the powder coating material 4. This filter element 50borders on the chamber 10, 12 between its inlet 14, 16 and its outlet18, 20 along part of its length in the circumferential direction andforms a section of the circumferential wall of the chamber 10, 12.

The hollow-cylindrical filter element 50 consists of sinteredpolyethylene with a wall thickness of 2-4 mm and a pore size of about 5μm and has an inside diameter of 5-30 mm, which basically corresponds tothe inside diameter of the adjoining wall sections 40, 42 on eitherside, the Y-shaped sections of line 22 and 26, and the feed line 24 andthe delivery line 28. The filter element 50 extends along about half thelength of the chamber (measured between the centers of the pinch valves30). At its two ends, the filter element is connected airtight with theadjoining sections 40, 42 of the circumferential wall.

As is shown best in FIGS. 3 and 9, each filter element 50 is surroundedby a housing 52, which is separated from the filter element 50 by acylindrical annular space 54. In the device shown in FIGS. 1 and 2, thehousing 52 has a connection 56, which can be alternately acted upon bynegative pressure from a negative pressure source 58 (FIG. 4) and bycompressed air from a compressed air tank 48 (FIG. 4). At the twoopposite ends of the housing 52 and the filter element 50, there is ahose connection 58, onto which the adjoining elastically pliablecircumferential wall section 42 can be pushed and fastened by hose clips(not shown). The hose connections 58 are screwed together with thehousing 52 by screw caps 60. Gaskets 62, 64 inserted between the hoseconnections 58 and the filter element 50 and the housing 52 preventcompressed air from escaping from the chamber 10, 12 or from the annularspace 54 of the housing 52, when compressed air is admitted to thechamber 10, 12 in this region, and prevent outside air from entering thechamber 10, 12 or the annular space 54, when negative pressure isapplied to the chamber 10, 12.

As is shown best in FIG. 4, the compressed air/negative pressureconnection 56 around the housing 52 of each conveying chamber 10, 12 isalternately connected by an electromagnetic 5-way control valve 66 withthe negative pressure source 58 and with the compressed air tank 48 toapply negative pressure to the given chamber 10, 12 to draw powdercoating material 4 out of the reservoir 6 with the inlet 14 or 16 openand the outlet 18 or 20 closed or to feed compressed air into thechamber 10, 12 to discharge the powder coating material 4 previouslydrawn into the chamber 10, 12 with the inlet 14 or 16 closed and theoutlet 18 or 20 open and convey it through the delivery line 28 by thedense phase or so-called plug flow method.

The compressed air tank 48, which can be filled with compressed air by acompressor 68, is connected with one of the three inlets of the 5-waycontrol valve 66 via a pressure regulator 70 and a throttle 72. Theother two inlets of the 5-way control valve 66 are connected by a line82 with the negative pressure source 58, which is designed as a vacuuminjector in the embodiment shown in the drawing.

As is shown best in FIG. 5, the vacuum injector 58 has an injectornozzle 74, which is supplied with compressed air P from the compressedair tank 48. During the feeding of compressed air into the injectornozzle 74, a negative pressure is produced by the venturi principle inan annular space 78 surrounding the outlet 76 of the injector nozzle 74.This negative pressure is applied via a connecting socket 80 and theline 82 to the two inlets of the 5-way control valve 66 that areconnected with the negative pressure source 58 and, with each switchingof the 5-way control valve 66, is alternately applied to one of the twochambers 10, 12, while, at the same time, the other chamber 12, 10 isbeing supplied with compressed air.

Instead of a 5-way control valve, a 4-way control valve may also beused, which has two inlets, one of which is connected with thecompressed air tank 48, and the other is connected with the negativepressure source 58, while each of the two outlets is connected with oneof the two chambers 10, 12, so that, with each switching of the valve,the chambers are alternately acted upon by compressed air and negativepressure.

At the same time that the 5-way control valve 66 is being switched, thetwo multiple-way control valves 44, 46 (FIG. 1) in the compressed airfeed lines of the pneumatic cylinders 32 are also being switched, inorder to close the outlet 18 or 20 and open the inlet 14 or 16 of thechamber 10 or 12 that has just been emptied and to close the inlet 16 or14 and open the outlet 20 or 18 of the chamber 12 or 10 that has justbeen filled.

As is shown best in FIG. 2, the application of a negative pressure Uwith the inlet 16 open causes powder coating material 4 to be drawn fromthe reservoir 6 and the feed line 24 into one of the chambers 12, whilethe compressed air P fed into the other chamber 10 through the filterelement 50 forces the powder coating material 4, which had previouslybeen drawn into this chamber 10, through the outlet 18 and through oneof the arms of the Y-shaped section of line 26 into the delivery line28. To ensure conveyance that is as frictionless as possible withoutlarge pressure losses, the Y-shaped section of line 26 and the Y-shapedsection of line 22 are curved at an angle of less than 30° at each oftheir bending points, i.e., at the connecting points between the leg andthe two arms of the Y and between each arm and the inlet 14, 16 of thecorresponding chamber 10, 12.

The compressed air fed into the given chamber 10, 12 during conveyancethrough the filter element 50 not only forces the powder coatingmaterial 4 present in the chamber 10, 12 into the delivery line 28, butalso cleans the cylindrical inside surface of the filter element 50 ofadhering powder coating material 4, which had previously been drawn ontothis surface by the previously applied negative pressure. It was found,however, that the pressure surge produced inside the chamber 10, 12 bythe admission of compressed air into the chamber 10, 12 is not alwayssufficient to also clean the remaining inner surfaces of the chamber 10,12 of powder particles, since the powder particles can adhere relativelystrongly to the surfaces of the chamber 10, 12 due to a triboelectriccharge.

To be sure, to clean a conveying chamber, for example, chamber 10, it isbasically possible, by suitable switching of the multiple-way controlvalves 44, 46 of the pneumatic cylinders 32, to open both the inlet 14and the outlet 18 of this chamber 10 (and to close the inlet 16 and theoutlet 20 of the other chamber 12), and then to blow out this chamber10, together with the delivery line 26 and the feed line 24 from thereservoir 6 or from the spray gun 8.

However, since this manner of cleaning the entire conveyance line is notalways desired, the two chambers 10, 12 are each equipped with anadditional cleaning valve 84 (FIGS. 2 and 9), through which compressedair can be blown into the interior of the chamber 10, 12 to clean thechamber. The cleaning valve 84, which opens radially into the chamber10, 12 from the side between the filter element 50 and the inlet 14,16(or the outlet 18, 20), consists essentially of a metallic pipe fitting86 with an annular cross section, which is screwed airtight into aninternally threaded bore of a pipe connection 88 projecting beyond thechamber wall 42 by means of an external thread on its thicker end facingaway from the chamber 10, 12, and whose thinner end facing the chamber10, 12 is closed at its end face and has several radial drill holes 90in the wall 92 of its cylindrical surface (cf. FIG. 6). A diaphragm inthe form of a hose coupling 94 made of an elastic rubber material isfastened on the thinner end of the pipe fitting 86. The diaphragm restsloosely against the peripheral surface of the pipe fitting 86 undertensile force and seals the drill holes 90, as shown at the bottom inFIGS. 2 and 9. During the admission of compressed air into the cleaningvalve 84, the diaphragm 94 is lifted from the outer peripheral surfaceof the pipe fitting 86 by the compressed air, so that the compressed aircan flow between the diaphragm 94 and this peripheral surface and intothe chamber 10, as shown at the top of FIGS. 2 and 9. Upon completion ofthe admission of compressed air, the deformed diaphragm 94 moves backagainst the peripheral surface of the pipe fitting 86 due to its elasticrestoring force and prevents powder coating material 4 from escapingthrough the cleaning valve 84 when compressed air is later admitted intothe chamber 10, 12.

In the enlarged view of the cleaning valve 84 shown in FIG. 6, a ballcheck valve 96 is provided in addition to the diaphragm. When compressedair is fed into the valve 96, the ball 98 of this valve is forced fromits seat against the force of a spring 102.

In contrast to the device 2 in FIGS. 1 to 6, in the device 2 in FIGS. 7to 10, the housing 52 of each filter element 50 is provided with twoconnections 104 and 106. While one of the connections 104 is locatednear outlet end of the filter element 50 and can be acted upon bynegative pressure, the other connection 106 is located near the inletend of the filter element 50 and can be acted upon by compressed air Pfrom the compressed air tank 48.

As is best shown in FIG. 10, a line 108 branches off for this purposebetween the five-way control valve 66 and the housing 52 of each filterelement, such that a spring-loaded nonreturn valve 114, 116 is insertedin opposite directions of installation into each of the two branch lines110 and 112 that lead to the connections 104 and 106, so that, whencompressed air is fed into the line 108, the nonreturn valve 114 locatedin front of the connection 104 opens against the force of the spring,while the nonreturn valve 116 located in front of the connection 106remains closed. On the other hand, when a negative pressure is appliedto the line 118, the nonreturn valve 116 opens against the force of thespring, while the nonreturn valve 114 remains closed.

In addition, to further increase the conveyance capacity of the device2, the hollow-cylindrical filter elements 50 in the device shown inFIGS. 7 to 10 have a greater length and a greater ratio of length toinside diameter, namely, a length of 80 and 250 mm, respectively, withan inside diameter of 6 and 12 mm, respectively, and a correspondingchamber length between the centers of the pinch valves 30 on the inletand outlet sides of 180 and 400 mm, respectively.

1. A system for delivering and spraying powder coating materials, comprising: a powder reservoir; a feed line connected to said reservoir; a conveyance device for the pneumatic conveyance of powdered material connected to said feed line; a delivery line connected to said conveyance device; wherein said conveyance device includes: a first conveying chamber and a second conveying chamber, said first conveying chamber comprising a first rigid gas permeable filter element encircled by a first annular housing, said second conveying chamber comprising a second gas permeable rigid filter element encircled by a second annular housing; a first inlet pinch valve at said inlet of said first conveying chamber; a first outlet pinch valve at said outlet of said first conveying chamber; a second inlet pinch valve at said inlet of said second conveying chamber; a second outlet pinch valve at said outlet of said second conveying chamber; a first Y-shaped line section having one inlet and two outlets, with said inlet connected to said reservoir through said feed line and said outlets connected to said first inlet pinch valve and said second inlet pinch valve respectively; a second Y-shaped line section having two inlets and one outlet, with said inlets being connected to said first outlet pinch valve and said second outlet pinch valve respectively, and said outlet connected to said delivery line; one or more control valves being operable in a first mode of operation to selectively open or close said first and second inlet pinch valves and said first and second outlet pinch valves, and to selectively apply negative pressure or admit compressed gas through said first and second rigid gas permeable filter elements to draw in powdered material from said reservoir through said feed line into said first conveying chamber while said second conveying chamber forces powdered material, which had been previously drawn into said second conveying chamber, out of said second conveying chamber through said delivery line; and to draw powdered material from said reservoir through said feed line into said second conveying chamber, while said first conveying chamber forces powdered material, which had been previously drawn into said first conveying chamber, out of said first conveying chamber through said delivery line; said one or more control valves being operable in a second mode of operation to clean powdered material from said first and second conveying chambers, said first and second inlet pinch valves, said first and second outlet pinch valves, said first and second Y-shaped line sections, said feed line, and said delivery line by operating said one or more control valves to selectively open said first and second inlet pinch valves and first and second outlet pinch valves while selectively admitting compressed gas through said first and second rigid gas permeable filter elements to allow compressed gas to flow through said first and second conveying chambers, said first and second inlet pinch valves, said first and second outlet pinch valves, said first and second Y-shaped line sections, said feed line, and said delivery line.
 2. The system of claim 1 wherein said first and second rigid gas permeable filter elements extend for more than one third of the length of the first and second conveying chambers, respectively.
 3. The system of claim 1 wherein said first Y-shaped line section has a leg connected to said inlet and two arms connected to said outlets, respectively, the angles between said leg and said arms being less than 30°, and wherein said second Y-shaped line section has a leg connected to said outlet and two arms connected to said inlets, respectively, the angles between said leg and said arms being less than 30°.
 4. The system of claim 3, wherein the first conveying chamber and the second conveying chamber each have a longitudinal axis, and wherein the angle between each of said arms of said first and second Y-shaped line section and the longitudinal axis of the chamber to which each of said arms is connected is less than 30°.
 5. The system of claim 1 wherein the length to inside diameter ratio of the first and second rigid gas permeable filter elements is 10 to
 30. 6. The system of claim 1 wherein a vacuum injector is used to apply negative pressure through said first and second rigid gas permeable filter elements.
 7. The system of claim 1 further comprising first and second cleaning valves which can supply compressed gas to said first and second conveying chambers, respectively, without passing said compressed gas through said first and second rigid gas permeable filter elements.
 8. The system of claim 1 wherein said first conveying chamber further comprises first and second connectors and first and second threaded screw caps, said first annular housing has first and second threaded ends, and said first rigid gas permeable filter element has first and second ends, said first and second ends of said first rigid gas permeable filter element being received within said first and second connectors respectively, said first and second connectors being received within said first and second end caps respectively, and said first and second end caps being threadably engaged with said first and second threaded ends respectively of said first annular housing to secure said first rigid gas permeable filter element within said first annular housing to create an annular space therebetween.
 9. The system of claim 8 wherein an inner gasket is supported on the inside surface of each of said first and second connectors and an outer gasket is supported on the outside surface of each of said first and second connectors, said inner gasket being compressed by said first rigid gas permeable filter element and said outer gasket being compressed by said first annular housing.
 10. The system of claim 1 wherein said outlet of said second Y-shaped line section is connected to a spray gun through said delivery line. 